Urban Warming Could Spell Trouble for Backyard Bees

Summer is here and like every summer, the temperature outside is punishingly hot.

In North Carolina, we just have to face facts that it is going to be hot everywhere from mid-May through September, but if you walk through downtown Raleigh, Charlotte or Greenville with cars slowly driving by, air conditioners humming and the sun beating down, you could mistake the feeling for being in an oven.

If you were to guess that it's hotter in the cities than everywhere else, you would not be wrong. In fact, environmental scientists have a name for that phenomenon: urban heat islands. In these islands, the average temperature can be up to five degrees higher than in surrounding rural areas, all because of human activities.

Elsa Youngsteadt, a research associate at NC State and co-lead author of the study examined tested the temperature tolerance of 15 bee species common in the Southeastern United States and how their populations related to that tolerance. Her findings, published in the journal, Biology Letters, show population declines across the board as the temperature rises.

To begin with the researchers gathered bees from 18 urban sites across Wake County to identify which species to study. They then brought each of the 15 species into a lab and put them in temperature-controlled tubes. The researchers raised the temperature in the tubes until the bees stopped moving all together, the “critical thermal maximum” or CTmax for each species.

The CTmax values hit 50 degrees Celsius at their highest, with carpenter bees squirming around at more than 120 degrees Fahrenheit. Bumble bees and sweat bees, however, stop twitching in the mid-40’s Celsius, or just over 110 Fahrenheit, and anyone who’s sat outside at a restaurant in mid-July knows the temperature on the pavement can easily pop 110 when the sun is beating down on a monstrously hot day.

Still, the heat tubes may show how the bees respond in a precise, controlled setting, but in the real world, where wind, shade, direct sun and humidity can play contribute to how hot it feels, the results can be a little messier. Youngsteadt and her colleagues, therefore, watched those 18 sights in Wake County for two years to determine how the populations of the bee species responded to rising urban temperatures.

As you might expect, bees with the lowest CTmax, and therefore the lowest heat tolerance, fared the worst as temperatures rose. Youngsteadt says these species are the most vulnerable to rising temperatures and that over long periods of time.